644 



ELECTRICAL METHODS 



[Chap. 10 



Fig. 10-10. Wheat 

 stone bridge arrange 

 ment for rock-resistiv 

 ity determination. ' 



is the "substitution" method illustrated in Fig. 

 10-8. A galvanometer deflection is first observed 

 and adjusted by means of the shunt to give a suit- 

 able deflection with the unknown resistance in 

 series. Then the switch is thrown over to the 

 known resistance, which is adjusted until the same 

 galvanometer deflection is obtained. The use of 

 this method with A.C. is shown in Fig. 10-12. 



For a determination of the resistance of bodies 

 of arbitrary shape, separate measurements of cur- 

 rent and voltage are made. Four connections are 

 then necessary for the current and voltage leads. 

 As H. V. Helmholtz pointed out as early as 1853, 

 the current leads may be interchanged with the potential leads without 

 altering the resistance. This value is equal to the ratio of voltage differ- 

 ence and current, multiplied by a factor controlled by the spacing of the 

 electrodes; the method is particularly suitable for measurements of 

 resistivities in situ. Since it employs commutated D.C., this procedure 

 will be discussed in the next paragraph. 



3. Methods using commutated D.C. have attained great practical impor- 

 tance for the determination of resistivity in the laboratory and in the field. 

 One reason is simplicity of technique. D.C. instruments may be used 

 throughout, since the current through the ground may be commutated in 

 synchronism with the current through the meters. In this category are 

 the well-known Wenner-Gish-Rooney and the "Megger" instruments. 



Doubli Commufator 



Eledredes making 

 conftef with ttrfh 



Pottntiomtttr 



MiWammettr 



Fig. 10-11. Resistivity determination by four-terminal (Gish-Rooney) method. 



The Wenner-Gish-Rooney method is illustrated in Fig. 10-11. It con- 

 sists of a set of B batteries, a double commutator, four electrodes, a milli- 

 ammeter, and a potentiometer. Current from the batteries passes through 



